SOP: Determination of Sodium Chloride in Products

GIFSQ — Global Institute for Food Safety & Quality

Principle

The chlorides in the product are precipitated by an excess of silver nitrate, and the organic matter is then oxidised with potassium permanganate in acid solution. The excess permanganate is decomposed by sucrose, and the unused silver nitrate is determined by titration with thiocyanate. The sodium chloride content is calculated from the amount of silver nitrate used.

Apparatus required

• 250-ml conical flask
• Glass beads
• Fume cupboard
• Burette (for the thiocyanate titration)

Reagents

a. Dilute nitric acid, HNO₃ (1 + 3).

b. 0.05 N silver nitrate, AgNO₃.

c. 0.05 N ammonium thiocyanate or potassium thiocyanate solution in water — standardize the solution accurately (prepared and standardized as in the assay of sodium chloride content).

d. 5% ferric ammonium sulphate in 10% HNO₃ — the indicator.

e. Saturated solution of potassium permanganate, KMnO₄, in water.

f. Sucrose.

g. Urea.

h. Acetone.

Sample preparation

Take a known weight of the product sample containing 30–50 mg of NaCl.

Procedure

Take the weighed sample containing 30–50 mg of NaCl in a 250-ml conical flask; add 30 ml of water and a few glass beads

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Warm the flask and pipette in 25 ml of 0.05 N AgNO₃ solution

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Add 10 ml of dilute HNO₃; boil gently in a fume cupboard for 5 min

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Cool to 80°C; with mixing, add KMnO₄ solution until there is a slight pink colour

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Boil gently until the colour disappears

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Add 0.5 ml of KMnO₄ and boil again; repeat this treatment until the solution is not decolourized after 5 min of boiling

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Add sucrose in small amounts and boil until the solution is colourless

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Cool for 30 sec; add about 0.1 g of urea; cool for 10 min in running water

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Add 5 ml of acetone and 2 ml of ferric ammonium sulphate indicator; titrate with 0.05 N thiocyanate solution to a pink end point

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Carry out a blank determination in the same way, omitting the sample

Calculation

The sodium chloride content is calculated from the silver nitrate consumed:

% Sodium chloride = (ml of thiocyanate in blank − ml of thiocyanate in sample) × N of thiocyanate × 58.44 × 100 ÷ (weight of sample in g × 1000)

Where,

• ml of thiocyanate in blank = titre of the blank determination
• ml of thiocyanate in sample = titre of the sample determination
• N of thiocyanate = normality of the thiocyanate solution
• 58.44 = equivalent weight of NaCl
• weight of sample in g = weight of the sample taken

Note: the book’s printed calculation line is garbled in the extraction; the formula above follows the printed variables (blank and sample thiocyanate titres, sample weight) in the standard form.

Precautions

• Boil gently in a fume cupboard during the permanganate oxidation.
• Repeat the permanganate treatment until the solution is not decolourized after 5 min of boiling.
• Cool the flask for the full 10 min in running water before titrating.
• Always run the blank determination alongside the sample.

References

Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.

Official Methods of Analysis, Association of Official Analytical Chemists, Washington, DC, 12th edn, p. 626, 1975 — cited in the book.